1. Field of the Invention
This invention relates to a process for producing steel wire or rods having high ductility and high strength, and more particularly to a process for producing such rods having a tensile strength greater than 100 kg/mm.sup.2 after rolling.
2. Description of the Prior Art
Increased strength in steel wire rods is generally achieved by forming a fine pearlite structure by means of a patenting treatment of a high carbon steel, followed by a wire drawing operation producing a large reduction of area. However, this method is applicable only to the production of wires of high strength and high ductility having small gages, since the ductility of the steel is influenced by the rod diameter at the time of patenting, and by the fact that the rods of larger gage can be wire drawn only to a limited extent while a large reduction of area in wire drawing is required for substantial enhancement of strength.
On the other hand, attempts have also been made to form a martensite structure, using a low carbon steel. A recent attempt has been made to produce a martensite structure by quenching a low carbon steel rod immediately after hot rolling in order to conserve energy.
However, such martensitic steel rods have the drawback that the quenched steel has a relatively low ductility and wire drawability, although a high strength can be achieved. The physical properties of martensitic steel rods of the prior art are shown in FIGS. 1-3. FIG. 1 illustrates the strength and ductility of a reheated and quenched wire rod (5.5 mm diameter) after water quenching, as a function of the C-content. It can be seen from this figure that increased strength of the martensite can be easily achieved by increasing the C-content, although the ductility deteriorates markedly and the reduction of area is decreased if the C-content exceeds 0.2%. Specimens having a C-content in excess of 0.25% fractured by yielding in a tensile strength test, and cracks extending along the length of the wire were clearly observed immediately after quenching when the C-content exceeded 0.35%. FIG. 2 shows the strength and ductility of a reheat-quenched wire rod which was subsequently tempered for one hour at 400.degree. C. As can be seen from the figure, the ductility of the quenched wire rod is clearly restored by the tempering, but this is accompanied by a substantial drop in strength.
In current practice, it is conventional to draw a quenched rod into wire after tempering. FIG. 3 shows the relationship between the reduction of area and the tensile strength when a reheat-quenched 0.14% C carbon steel (a wire rod of 3.1 mm diameter) having a tensile strength of 132 kg/mm.sup.2 after quenching, is subjected to wire drawing after restoration of ductility and wire drawability by tempering (tensile strength after tempering: 102 kg/mm.sup.2). This figure also shows the relationship between the reduction of area and the tensile strength in a wire drawing operation for 0.8% C high carbon steel (a wire 5.5 mm in diameter) after patenting at 550.degree. C. It can be seen that with martensitic steel wire which has been tempered for restoration of ductility and wire drawability, it is difficult to attain a strength comparable to that of the conventional high carbon steels. Any improvement in ductility by tempering a quenched wire rod seems to be related to a decrease in strength. That is, ther is an inverse relationship between the ductility and the strength in martensitic steel wire rods. The fracture stress, .sigma..sub.f (true stress at the time of fracturing=fracturing load/area of fractured surface), which indicates a balance between ductility and strength, is about 170-190 kg/mm.sup.2 for a quenched or tempered martensitic steel having a carbon content higher than 0.2%. Therefore, even if the carbon content is increased for the purpose of enhancing the strength, the ductility decreases with increasing strength within the range of constant .sigma..sub.f. This can be seen in FIGS. 1 and 2. Consequently, in order to find a practical utility for high strength quenched steel having a carbon content greater than 0.2%, it is necessary to enhance the value of .sigma..sub.f by improving the essential properties of the martensite itself.
It can be seen from FIG. 4, which shows the relationship between the carbon content and the martensite transformation temperatures Ms=starting temperature; Mf=finishing temperature), that the transformation temperatures are lowered as the carbon content is increased. It is known in the art that cracking occures when steel of a low transformation temperature is quenched.
Accordingly, a need has continued to exist for steel wire and rod of a martensitic structure which has both high strength and good ductility.